1. Field of the Invention
This invention relates to a circuit for combining electronic signal power from multiple sources such as amplifiers or for dividing a single signal and providing it to multiple loads, such as amplifier inputs. The invention has particular applicability to radio frequency (RF) power circuits.
2. Description of the Related Art
The present invention arose in the field of RF transmission, where power requirements may exceed practical levels for a single amplifier, making it desirable to amplify a signal with multiple RF power amplifiers in parallel. An input power divider circuit is used to divide the signal and provide it to the inputs of the multiple amplifiers, while an output combiner provides the combined outputs of the multiple amplifiers to the next stage load. The present invention is an improved circuit that will be discussed mainly as an output combiner, for simplicity, but which can also be arranged as an input divider, as will be shown.
An important attribute of an output combiner is its capability to provide the correct load impedance to the outputs of the amplifiers (connected to its input ports), when the combiner is terminated in its actual load impedance. That is, the input impedance of the combiner, when it is terminated in the actual load impedance, should be that known impedance which allows the amplifier to deliver full rated power.
A second attribute of a combiner is its ability to provide isolation between the outputs of the amplifiers connected to its input ports, preventing power from one amplifier from being reflected back into another amplifier.
A third important attribute of a combiner is its ability to maintain the correct impedance and isolation if there is a failure or removal of one or more of the amplifiers connected to it. That is, the impedance presented to the operative amplifiers should continue to be the correct value, and the amplifiers should continue to be isolated from each other.
Combiner designs vary in their abilities to provide the above attributes, particularly in their responses to amplifier failures.
Two conventional combiner designs are shown in the Motorola RF Data Manual, first edition, Motorola, Inc. 1978, in a first article entitled "Broadband Transformers and Power Combining Techniques for RF", beginning at page 2-51 and a second article entitled "A Two-Stage 1KW Solid-State Linear Amplifier", beginning at page 4-31, both prepared by H. O. Granberg. These designs are discussed in more detail in a following section of this application, in connection with figures, so that they may be clearly contrasted with the present invention.
The combiner described in the first article provides good isolation between amplifiers and maintains correct load impedance if one or more amplifiers become inoperative. However, this design has some disadvantages: (a) it can only be used to combine a number of amplifiers which is a power of two, (b) the transformers in this design are arranged in stages and must be able to handle larger power levels, the closer their stage is to the output of the combiner, and (c) the design requires a final and, often, intermediate transformer solely for the purpose of stepping back up to the proper impedance level.
The design of the second article reference above overcomes disadvantages (a) and (b) of the first conventional design, but not (c). In addition, it has a balance resistor that can only be optimized for a predefined number of failed amplifiers. For other than the predefined number, the balance resistor is not optimum and results in an incorrect impedance being presented to the amplifiers and reduced isolation between the remaining active amplifiers.
Another aspect of combiner performance is the maximization of power flow through the combiner in case there is a failure of one of the amplifiers connected to it. Even when the combiner maintains good isolation and continues to present the correct impedance to the operative amplifiers after the failure, the absence of an amplifier generally causes operation in which significant power is dissipated in balancing resistors within the combiner. In the conventional designs, there are not straightforward adjustments that will permit the combiner to pass through to its load all the power received from the operative amplifiers. Instead, these designs would require extensive circuit reconfiguration and/or a change in the value of one or more balancing resistors.